Piezoelectric devices find many applications as electro-mechanical actuators and transducers. Piezoelectric devices may be further classified as either employing a bulk piezoelectric material, or a piezoelectric thin film. A bulk piezoelectric material generally has a thickness over 20 microns (μm), and often well over 50 μm, while thin film devices employ a piezoelectric membrane that is less than 15 μm in thickness.
Generally, physical displacement of a piezoelectric material in the presence of an electric field of a given strength is a function of the thickness of the piezoelectric film. One technique in the art to increase displacement in a bulk piezoelectric device 100 is depicted in FIG. 1. The bulk device 100 utilizes a plurality of bulk piezoelectric material slabs 135A, B, C (e.g., with T1 of each slab being >50 μm in (z) thickness) stacked up in an alternating manner with a plurality of electrodes 110, 120, 130, 140. As further shown in FIG. 1, voltage biasing of the electrodes is such that the electric field applied across each successive piezoelectric slab is in an opposite direction as that of the subjacent and superjacent slab. For example, at a given instant in time a positive voltage is applied to electrodes 110 and 130 while a negative voltage is applied to electrodes 120 and 140. This biasing may be used first to pole the bulk piezoelectric slabs into opposing polarization (e.g., slabs 135A and 135C having polarization P1 in a first direction, and slab 135B having a polarization P2 in second direction opposite P1 and P3). This same biasing may then be used to impart a time varying electric field across the slabs that induces a large cumulative physical displacement.
Thin film piezoelectric devices can be advantageously fabricated inexpensively to exceedingly high dimensional tolerances using various micromachining techniques (e.g., material deposition, lithographic patterning, feature formation by etching, etc.). As such, one or more piezoelectric thin film device may be fabricated into Microelectromechanical systems (MEMS) that may further include one or more integrated circuit (IC) fabricated with compatible techniques. As one example, a microfluidic device, including one or more fluidic chambers and piezoelectric pumping actuators, can be formed in a single printer head die. As another example, an ultrasonic transducer, including an array of piezoelectric membranes capable of generating a high frequency pressure wave in a propagation medium (e.g., air, water, or body tissue) in contact with an exposed outer surface of the transducer element, can be formed in a single MEMS transducer die.
One issue with conventional thin film piezoelectric materials is that the thickness of piezoelectric thin film material may be limited by one or more aspects of the thin film fabrication process (e.g., film deposition constraints). Thus, the relationship between displacement of the piezoelectric material in the presence of an electric field of a given strength and the thickness of the piezoelectric film may not be readily exploited in the same manner as for the bulk piezoelectric device 100. As such, limitations on the thickness of the piezoelectric thin film can limit the performance of a thin film piezoelectric device. Structures and techniques that enable greater piezoelectric thin film thicknesses, and therefore enable greater degrees of freedom with respect to designed membrane displacement, are therefore commercially advantageous.